A general rule to go by to get the maximum performance out of a given muzzle energy is to try and get your bb's muzzle velocity as close to 300 fps regardless of muzzle energy. There are complex reasons why this is true but for example if your gun outputs 1J you should be using .25 for the best results for 1.5J you should be using .36 etc. However, since the difference is marginal, feel free to use whatever you choose, just pick a good brand.
For more reading
http://bbbastard.com/msed/